1. Field of the Invention
The present invention relates to a control for a vehicle power transmission and more particularly to a control for a steplessly variable vehicle power transmission.
2. Description of the Prior Art
It has been known to control the speed ratio of asteplessly variable vehicle power transmission in accordance with vehicle operating conditions. For example, the U.S. Pat. No. 4,161,894 discloses a vehicle transmission including a belt-pulley type steplessly variable transmission mechanism provided with a hydraulic actuator for adjusting the speed ratio of the transmission mechanism. A speed ratio control valve is provided for adjusting the hydraulic pressure applied to the actuator in accordance with the vehicle operating conditions so that a desired speed ratio can be obtained. For the purpose, that control valve has a valve spool, the position of which is determined by the engine intake pressure and a pressure representing the speed ratio of the transmission mechanism. The known transmission further includes a selector valve for modifying the pressure applied to the actuator in accordance with the position of an engine control foot pedal so that the speed ratio adjusting characteristics can be changed depending on the engine output.
It should, however, be noted that the above-described transmission system cannot satisfy demands in automatic transmissions for modern passenger cars. For example, the control system as proposed by that U.S. patent cannot provide adequate acceleration under partial throttle valve opening.